Light weight sandwich structures are used in structural applications such as vehicles, aerospace industry, framework etc. because of their superior strength and stiffness properties along through-the-thickness direction under bending loads. The use of inserts is essential to strengthen the sandwich structures to withstand localized loads. Further, when the external members or sub-structures are attached to sandwich structures, inserts become a necessity.
The specific strength of an insert assembly is a ratio of load at failure initiation to weight of the insert assembly which should be as high as possible to achieve effective utilization of sandwich structures with inserts. In practice, these inserts are made of aluminum alloys, other metals/alloys etc. High density of metals/alloys increases the weight of insert assembly resulting in undesirable reduction in the specific strength. Further, the difference in material properties at the interface between the insert and the potting material such as resin leads to high stress concentration at the interfaces between insert and potting material and between potting material and core.
U.S. Pat. No. 5,240,543 discloses a basic procedure to seat a fastener insert in a honeycomb panel. A fastener insert made of alloy attached to a mounting fixture is inserted into a hole drilled in a honeycomb panel. The mounting fixture has a rod with a disc at an end descending from a base and a chimney ascending from the base. Two bores are located in the base and are enclosed by the chimney. The mounting fixture base overlaps the head of the fastener insert which has two bores in the head. The bores in the fastener insert head are aligned with bores in the base of the mounting fixture. Epoxy is inserted into one of the bores in the base of the mounting fixture continuously until it returns out of the second bore. The epoxy is allowed to cure and thereafter the mounting fixture is removed by a turning motion. Though this patent describes the method of installation of inserts into honeycomb sandwich structures, it does not address the issue of minimization of the insert assembly weight and a means to increase specific strength.
U.S. Pat. No. 5,082,405 and U.S. Pat. No. 4,941,785 disclose the geometry of the inserts and the method to attach the insert to the attachment. In this insert assembly, epoxy resin is used as potting compound. The insert is a metallic member, made of stainless steel, aluminum alloy, and carbon steel. However, a metal insert increases the weight of the insert assembly resulting in reduction of specific strength.
U.S. Pat. No. 5,053,285 discloses the method and apparatus for making corrugated aluminum inserts. These inserts are made from aluminum foil strips by passing the strips through the corrugating device. Though the weight of such inserts is less, stresses at the interfaces of different materials increase because the elastic properties of corrugated aluminum inserts are less resulting in higher stress concentrations.
U.S. Pat. No. 6,055,790 discloses construction of an insert wherein the conduction and radiation are improved through the inserts. To improve the heat transfer rate, one of the face-plates is made of metal sheet. The insert material is aluminum alloy. In this arrangement, a higher thermally conducting path is provided from one side of the insert assembly to the other side. However, these inserts suffer from the deficiency such that the surrounding potting material made of resin which is not a good conductor of heat that leads to thermal gradient along the radial direction. This causes higher thermal stresses in the sandwich structure. The use of metal for face-plate increases weight of insert assembly thereby decreasing the specific strength.
U.S. Pat. No. 3,271,498 discloses an improved method of installation of inserts. However it does not address the issue of weight of an insert assembly.
A method of fabricating a honeycomb core structure with embedded fastener is disclosed in U.S. Pat. No. 4,716,067. The bonding material is epoxy resin. The method comprises laying down a first nonmetallic synthetic layer, which may be cured or uncured. Bonded to the first nonmetallic synthetic layer is a honeycomb core layer into which one or more flush head fasteners are inserted into holes in the honeycomb core that have an indentation at one end configured to mate with the underside of the head of a flush head fastener. Bonding is accomplished by priming the walls of the honeycomb core layer adjacent the surfaces of the layer, and the fastener, with a suitable primer, preferably before the fastener is installed, and, coating one of the surfaces of the first nonmetallic synthetic layer with a suitable adhesive before joining the first nonmetallic synthetic layer to the surface of the honeycomb core layer containing the head of the flush head fastener. Thereafter, the cells of the honeycomb core that surround the fastener are filled with a potting material. Then, a second nonmetallic synthetic layer is bonded to the surface of the honeycomb core remote from the surface to which the first nonmetallic synthetic layer is bonded. There are several drawbacks of this method and insert assembly. The fasteners are made of an alloy. The density of the alloys used is more compared to the composites. Significantly high interfacial stresses develop between the alloy insert and the potting resin material. Further, this patent describes the method of installation of inserts into honeycomb sandwich structures. It does not address the issue of enhancing the specific strength of insert assemblies.
The shortcomings in prior art vis-a vis use of inserts in sandwich structure are                Use of metal for inserts which leads to decrease in specific strength of the insert assembly.        Though geometry of the inserts and different methods of installation of these inserts into sandwich structures is addressed, the issue of minimization of the insert assembly weight and a means to increase specific strength of insert assembly has not been addressed.        There are no methods of reliably mapping stress distribution and prediction of failure initiation in sandwich structures with diverse geometrical configurations.        